1. Field of the Invention
The present invention relates to methods for the control of internal combustion engines with turbochargers and exhaust gas recirculation.
2. Background Art
To improve performance, many internal combustion engines, particularly diesel engines, include a turbocharger to increase the oxygen density of the cylinder charge. Turbochargers use the engine exhaust gases to operate a turbine which in turn powers a compressor to compress intake air. Variable geometry turbochargers (VGT), which include variable nozzle turbochargers (VNT), are used to modify turbocharger characteristics over a broader engine operating range than that possible with conventional turbochargers. Moveable intake or exhaust vanes (VNT) or a moveable turbine sidewall may be used to provide an appropriate amount of turbo boost pressure for current operating conditions and driver demanded engine torque.
Exhaust gas recirculation (EGR) has known advantages with respect to reducing emissions of nitrogen oxides (NOx) by reducing peak combustion temperatures within the engine cylinders. EGR systems typically include an EGR valve which diverts engine exhaust gases from the engine exhaust manifold to the engine intake manifold. This requires a pressure differential between the exhaust and intake manifolds for the gas to flow from the exhaust to the intake. Naturally aspirated engines typically create a vacuum at the intake and have a considerably higher exhaust gas pressure which easily flows any desired amount of EGR. For turbocharged engines, additional back pressure may be required to provide a sufficient pressure differential to introduce the exhaust gas into the compressed intake air. One method for controlling back pressure is to modify the geometry of a VNT or VGT.
To achieve the benefits associated with EGR, both the EGR system components and VGT system components should be in proper working order. Various diagnostics have been developed to monitor the EGR system and VGT system. One such system is described in U.S. Pat. No. 6,457,461 to Romzek. Diagnosable malfunctions include both sensor failures and mechanical faults. Typically, EGR and VGT control systems respond to sensor failures by substituting stored default values for continued operation. However, because these stored default values are inflexible imperfect approximations, emissions reduction and engine efficiency can suffer greatly. When mechanical faults occur, continued error compensation attempts by the engine control logic typically force the variable geometry mechanism into either a completely open or completely closed position, also causing emissions reduction and engine efficiency to suffer.